The present invention relates to a device for measuring magnetic field and more particularly for measuring direct current of a primary conductor. The field of the invention is that of measuring magnetic fields (magnetometers) or measuring electric currents via their magnetic fields (contactless current transducer).
In the field of DC current sensors “shunts” are known, which have a high immunity to disturbing currents and allow the accurate measurement of currents, but they are not naturally isolated. The use of a shunt requires an electronic isolation which is generally expensive and bulky, not very robust under environmental stress (in particular high temperature). For high voltage applications (>50V), optical isolation solutions exist. Generally, the material used for the measurement is a material the resistivity of which has a substantially zero thermal drift constant (constantan for example). Sometimes copper is used directly; then a compensation for the thermal drifts must be provided, which complicates the measurement and generally makes it less accurate. Among the defects of the shunt, the dissipation of power by Joule effect can be noted. For a 1000 A sensor there is typically a resistance of 10 μOhm and therefore 10 W of heat dissipation. One of the consequences is the great difficulty of using a shunt in a compact system due to its self-heating.
Generally a shunt is also quite bulky so as to allow for exactly this temperature dissipation without exceeding the maximum temperature of the latter.
The open-loop Hall Effect sensors are not very accurate and are very sensitive to disturbing currents. In order to improve their immunity, magnetic shieldings are often used which introduce additional defects: they are subjected to strong magnetic remanences which decay over time and with temperature.
The zero-flux Hall effect sensors are very accurate and are based on the use of a magnetic core which acts as magnetic shielding and field channel. They are however also subject to decay of magnetic remanence over time and with temperature. They are also large and have a large space requirement due the presence of the core of magnetic material necessary for channelling the magnetic flux and for shielding.
Flux Gate technology is very sensitive and accurate but is extremely sensitive to crosstalk and to disturbed environments due to the high permeability of its magnetic material. The technology then requires heavy and bulky shielding solutions in order to prevent local saturation of the transducer core under the effect of a magnetic field.
The GMR (Giant MagnetoResistance) and/or AMR (Anisotropic MagnetoResistance) technologies are relatively sensitive and accurate when they use the zero flux principle. They are easily incorporated into an integrated circuit, except for the compensation coil which poses a problem for strong fields. As a result, they are relatively sensitive to crosstalk phenomena and they also show a magnetic offset drift due to the ferromagnetic nature of their transducer.
Fibre optic current sensors (FOCS) are based on the Faraday effect. They are very effective for measuring very high currents (up to 600 kA). They have an excellent immunity to crosstalk, and excellent performance during zero-flux operation. This introduces a high power consumption for high currents. However, a priori, FOCS cannot be utilized where integration is required, as they are relatively bulky and need to surround the primary conductor.
Current sensors of the transformer type (air-core Rogowski or magnetic-core current transformer (CT)) have a relatively low sensitivity to crosstalk, but do not allow DC components to be measured.
Moreover, for most of the technologies used, sudden variations in the primary voltage (dV/dt) cause injections of stray currents affecting the electronics of the sensor (at best saturation of the measurement stages, at worst destruction), that can be prohibitive. This is particularly noticeable in the case of the shunt. Certain technologies such as the transformer (CT, Rogowski or Hall or Flux Gate) allow the use of an electrostatic screen, as the measurement of the magnetic field is carried out in a core outside the primary conductor.
The Néel® Effect is very accurate. The technology described in patent No. FR 2891917 is called the Néel® Effect and is based on the use of a coil and a zero-remanence magnetic composite B(H) of which the third derivative at the origin presents an extremum (for example a superparamagnetic composite). The Néel® Effect technology is not very sensitive to external fields due to the low permeability of their magnetic material. They exist in a flexible “universal” form of Rogowski type for measuring direct currents (patent FR 2931945). This topology has a high immunity to crosstalk, taking Ampere's theorem as a basis and measuring the circulation of the magnetic field over a substantially closed external contour. This topology has the same advantages and drawbacks as the other technologies from the point of view of incorporation: they are large as they must surround the primary conductor and they have a high electricity consumption in order to operate at zero flux.
Compact topologies for Néel® Effect sensors have also been to described, based on a measurement directly on a busbar or even inside a busbar, in order to allow a significant reduction in consumption despite zero-flux operation (French patent applications No. 1158584 and No. 1162100). It is however difficult with these topologies to be able to ensure good voltage stability, and also to add an electrostatic screen and to have a high immunity to crosstalk.